In order to inject a fluid medicament into a patient when using a hypodermic syringe, three separate and distinct tasks must be performed. These are: 1) insertion of the needle into the patient; 2) injection of the fluid medicament from the syringe into the patient; and 3) withdrawal of the needle after the injection has been completed. For each task, the magnitude and direction of forces on the syringe, as well as the location of their application, are different from the other tasks. For instance, compare the task of inserting the needle, with the task of injecting the fluid medicament. Insertion of the needle requires that only minimal forces be applied on the syringe, and that they be applied for only a very short period of time. On the other hand, injection of the medicament requires a much greater force be applied. Further, this force must be applied on the plunger of the syringe for what will typically be a relatively longer period of time. In comparison with both of these tasks, needle withdrawal requires the application of a force in the opposite direction. These, and other similar considerations, become important when the injection process is to be automated.
Springs for generating forces on a syringe in an automated process have been used heretofore for various purposes. A characteristic of springs, however, is that the magnitude and direction of a spring force are not variable. Consequently, springs do not lend themselves for so-called “multi-tasking” operations. This is particularly so where precise control over a syringe injection operation is required, and different magnitude forces are sequentially required in the same direction (e.g. needle insertion and medicament injection).
In addition to the mechanical considerations mentioned above, the design of an auto-injector also requires “user-friendly” considerations. In particular, it is desirable that the injection needle of a syringe be operationally concealed from the view of a user. Preferably, this concealment can be maintained before, during and after an injection procedure. Further, it is desirable that operation of the syringe be limited to only those times when the syringe is properly positioned for an injection.
In light of the above, it is an object of the present invention to provide a two-motor device for performing the injection of a fluid medicament into a patient wherein each motor generates different forces on a hypodermic syringe for different purposes. Another object of the present invention is to provide a reusable injector that can be operationally engaged with a disposable, pre-filled syringe. Still another object of the present invention is to provide an auto-injector system wherein the needle of a pre-filled syringe is operationally concealed and the system is operable only when the injector is properly positioned against the skin of a patient for an injection. Another object of the present invention is to provide a system and a method for automatically injecting a fluid medicament from a pre-filled syringe that is relatively simple to manufacture, is easy to use and is comparatively cost effective.